This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 101 15 099.7, filed on Mar. 27, 2001, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method and a circuit arrangement for limiting the amplitude of an output signal of an inverting differential amplifier using two threshold value switches.
2. Background Information
In order to reduce the power consumption of integrated circuits, such circuits are being operated using ever smaller supply voltages. In this context, in the field of signal processing, the amplitude level is also being ever further limited. For example, in the field of wireless communication, since several signal stages are connected one after another in a circuit, the amplification of each individual stage may not become too great, so as not to overdrive or overload the signal input of the next successive amplifier stage. Therefore, it is necessary to limit the amplitudes of the output signals of the amplifier stages, also in connection with small voltages, which may even be below a typical diode voltage, for example. Simultaneously, the measures provided for limiting the amplitude should only slightly, if at all, increase the power consumption and shall not influence the other signal stages. An important field of application of such signal processing amplifier stages is the field of mobile and cellular telephones.
In conventional known methods, the amplitudes of an output signal are limited by means of a fixedly set negative or reverse feedback threshold. Examples of such conventional circuits and methods are represented in the publication by Jacob Millmann and Armin Gabel, xe2x80x9cPrecision Limiting, 2nd Editionxe2x80x9d, page 763, and in the publication by Dieter Nuehrmann xe2x80x9cDas Grosse Werkbuch Elektronikxe2x80x9d, 7th Edition, pages 1905 to 1909, whereby the amplitude limitation is only carried out for negative amplitudes above voltages of one volt.
A disadvantage of the previously known methods is that the level of the amplitude limitation is prescribed and established in a fixed manner during the process of fabricating the integrated circuit. Thus, the level of the amplitude limitation is not changeable by the end user with a reasonable or acceptable effort. Moreover, the amplitude cannot be limited for small signal voltages that are below a typical diode voltage, without loading the signal output in a low-resistance manner. Furthermore, in the conventional methods and circuit arrangements, the amplitude limitation is not symmetrically adjustable.
In view of the above, it is an object of the present invention to provide a method in which both the positive and negative amplitudes of an output signal of an amplifier can be limited, in an adjustable manner, even at voltages below 1 volt. It is another object of the invention to provide a circuit arrangement for carrying out such a method, which may be simply and economically fabricated. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in a method for limiting an amplitude of an output signal of an inverting differential amplifier, using two threshold value switches and a control unit. Especially according to the invention, the method includes the following features. The negative amplitude of the output signal is limited so as not to range below a first threshold value by means of a first threshold value switch, which is arranged in a first negative or reverse feedback branch of the differential amplifier. The positive amplitude of the output signal is limited so as not to range above a second threshold value by means of a second threshold value switch, which is arranged in a second negative or reverse feedback branch of the differential amplifier. The two threshold values are adjusted or set by means of a control signal of the control unit.
The above objects have further been achieved according to the invention in a circuit arrangement with a control unit and a differential amplifier having first and second negative feedback branches or loops for limiting the positive and negative amplitudes of the output signal. According to the invention, the circuit arrangement includes the following features. A respective threshold value switch is provided in each one of the two negative feedback branches. Each respective threshold value switch comprises an adjustable current source, a resistor, and a transistor. In each threshold value switch, the adjustable current source is connected to a respective supply voltage, the resistor is connected to the output of the differential amplifier, and the adjustable current source and the resistor are connected in series with each other so as to form a voltage divider. The output of this voltage divider between the current source and the resistor is connected with the control input terminal of the transistor, e.g. the base of a bipolar transistor or the gate of a field effect transistor. In one embodiment, the transistor selectively connects or disconnects the input of the differential amplifier with the supply voltage, while in another embodiment, the transistor selectively connects or disconnects the input of the differential amplifier with the output of the differential amplifier. The adjustable current source is connected with the control unit, which is further connected to a signal line for the transmission of programming commands.
The basic concept of the invention is to limit an output signal of an inverting differential amplifier in a high-resistance manner, in that the positive amplitude of the output signal is limited so as not to range above a first threshold value by a first threshold value switch in a first negative feedback branch of the inverting differential amplifier, while the negative amplitude of the output signal is limited so as not to range below a second threshold value by a second threshold value switch in a second negative feedback branch of the differential amplifier, whereby the two threshold value switches can be adjusted by means of a control signal from a programmable control unit. In order to provide the threshold values used for the amplitude limiting, by means of a simple circuit arrangement, it is advantageous if the inverting differential amplifier and the two threshold value switches are respectively supplied with a symmetrical supply voltage. It is further advantageous if the two threshold values have the same magnitude, in order to thereby ensure a symmetrical limitation of the amplitudes. This further prevents any possible falsifications, i.e. alterations or corruptions due to non-symmetrical clipping of the signal form in the signal processing.
According to a further embodiment feature of the method of the invention, the two threshold values in the two threshold value switches are generated by means of two controllable current sources. Since the current sources are very high resistance elements, a threshold value generated in this manner is only minimally dependent on a variation in the supply voltage. The current source is further connected with a resistor to form a voltage divider, so that variable and selectable voltage values can be adjusted at the output of the voltage divider, depending on the level of the supplied current. In an advantageous manner, these voltage values can be applied to the control input terminal, e.g. the base, of a transistor, which is arranged in common with the respective resistor in the respective negative feedback branch, for example, in order to thereby provide an offset voltage by means of which very small threshold values can be set for the amplitude limitation. Thereby, depending on the magnitude and direction of the current being supplied or injected by the respective current source, merely a small voltage at the signal output of the differential amplifier is sufficient in order to limit the positive and negative amplitudes of the output signal. It is especially advantageous if the transistor arranged in the negative feedback branch is connected between the output signal line and the input signal line of the amplifier. Thereby, especially in connection with pulse-form input signals, an interspersal over the supply voltage in the subsequent signal stages is prevented.
In a different further embodiment of the inventive method, the two controllable current sources are, for example, respectively embodied as complementary current mirrors, and are coupled with one another by means of a coupling element, for example a coupling transistor. In this regard, a reference current generated by a control unit is reflected or coupled by means of the coupling transistor from the first current mirror to the second current mirror. Thereby, both threshold values are simultaneously adjusted and set in common with one another using a single control signal, whereby the two threshold values exhibit a comparable temperature variation or dependence. Thus, the adjusted or selected symmetry in the limitation of the amplitudes of the output signal on the positive side and on the negative side will only be slightly, if at all, influenced by temperature variations.
In yet another different embodiment feature of the inventive method, it is advantageous to apply a reference voltage to the non-inverting input of the differential amplifier, whereby this reference voltage is derived from the average of the voltage of the input signal. Thereby, an amplification of direct (DC) voltage offsets of the input signal is reliably suppressed, which is advantageous, because this would otherwise lead quickly to an unsymmetrical limitation of the output signal.
Experiments and investigations conducted by the applicants have shown that the inventive method may especially advantageously be utilized for limiting the amplitude of the output signal of the differential amplifier in a xe2x80x9csoftxe2x80x9d manner, i.e. by so-called xe2x80x9csoft-clippingxe2x80x9d. Through such a soft application of the amplitude limitation, no interfering harmonics will be added to the output signal, which would otherwise very drastically interfere with the useful signal in view of the low amplitude levels.